The electric arc furnace (EAF) process is a common steel making practice in use today. In a typical EAF process, solid charge ingredients including raw scrap, limestone, burnt lime, iron ore and ferro-alloy additives are placed in the top-charge furnace unit.
A conventional furnace unit is equipped with (1) a roof lift and swing arrangement which permits the roof to swing aside when cold scrap is charged into the furnace; (2) a rocker and rail tilting type arrangement which permits the furnace to tilt forward for tapping and backward for slagging; (3) a system for additions through the furnace roof; and (4) evacuation systems for the removal of dust generated during the steel making cycle.
Electrodes are supported by electrode arms and clamps, and project from overhead down through the furnace roof. An electric arc surging between the electrodes and through the furnace charge, typically comprising largely scrap metal, produces heat which melts the charge and refines the steel. The molten steel is tapped, typically at about 3000.degree. F., into a ladle and cast into blooms or poured into ingot molds.
In such a process, particulate emissions are generated during (1) charging of scrap, (2) tapping of furnaces, (3) pneumatic injection of additives, (4) oxygen blowing, and (5) meltdown/refining periods. This particulate, which is individually and collectively referred to as electric arc furnace dust (hereinafter EAFD), is typically collected either as a dry waste in baghouses, or wet, as sludge.
The Environmental Protection Agency (EPA) has classified EAFD as EPA Hazardous Waste No. K061 (emission control dust/sludge from the primary production of steel in electric furnaces). EAFD contains such hazardous constituents as lead, cadmium, and chromium. In order to remove EAFD from the list of hazardous wastes, it must be transformed into a product which does not exceed the following leachate concentration limits (based on the Toxicity Characteristic Leaching Procedure (TCLP) as found in Appendix II of 40 C.F.R. .sctn. 261, which is incorporated herein by reference):
______________________________________ Arsenic 0.315 mg/l (maximum) Barium 6.3 mg/l Cadmium 0.063 mg/l Chromium 0.315 mg/l Lead 0.315 mg/l Mercury 0.0126 mg/l Selenium 0.063 mg/l Silver 0.315 mg/l ______________________________________
Several methods of chemically stabilizing EAFD have been disclosed. For example, U.S. Pat. Nos. 4,840,671 and 4,911,757 to Lynn et al. disclose a method and a mixture for stabilizing EAFD and similar dusts. The method disclosed in these two references include mixing EAFD with fly ash, lime, and water, among other ingredients. This method relies primarily on the pozzolanic characteristics of fly ash to physically entrap the hazardous constituents of EAFD within a cementitiously hardened product. Neither reference even implies the possibility that EAFD is itself pozzolanic. In fact, the contrary is indicated by the apparently necessary inclusion of fly ash.
In these known systems (of the aforementioned Lynn et al. patents), fly ash is added to the mixture. While the presence of fly ash does provide a pozzolanic set with a reduced permeability, there are operational negatives to using fly ash as an additive. These operational negatives include:
1. a requirement of a fly ash tank or silo and feeding system as part of the processing facility, which is a significant capital cost; PA0 2. an increase in facility operating cost because of the expense of purchasing fly ash; PA0 3. a requirement of additional work in quality control to ensure that the fly ash quality is acceptable for the process; and PA0 4. an increase in volume and in weight of the mixture during treatment, and (as discussed below) in the material ultimately to be placed for final disposal.
The EPA has specifically expressed concern over volume increase in the stabilization of wastes. The addition of fly ash to the composition increases the ultimate weight and volume of the composition. This in turn requires more landfill space. If processed EAFD is to be disposed of off-site, the disposal cost is increased by an amount proportional to the increased weight or volume.
For the reasons stated, eliminating fly ash from the composition would be beneficial to the economics of the process.
Prior efforts to dispose of EAFD include "greenballing," a term which refers to pelletizing EAFD with water, usually in a pan pelletizer. The primary purpose for greenballing is to transform the EAFD from a dust to an agglomerate more suitable for immediate recycling back into the furnace. Also, greenballing has been used to transport the EAFD for disposal at another cite. In some cases, excessive lime was added, which led to expansion and cracking. In other cases where the lime content was too low, the introduction of rain water caused failure. Any possible cementitious reaction of the EAFD, lime and water mixture was neither recognized nor utilized.